1. Field of the Invention
The present invention relates to a process for preparing a single oxide crystal, and particularly to an improved process employing the flux method for growing a single oxide crystal of high quality.
2. Description of the Prior Art
The flux method comprises the steps of dissolving raw materials, from which a single crystal is to be grown, in an adequate flux to prepare a high temperature saturated solution, and cooling the solution slowly to precipitate and grow a single crystal. The flux method can advantageously be applied to a very wide variety of raw materials, and enables a high melting point single crystal to be grown at a temperature far lower than the melting point thereof. In addition, growth of the single crystal can be done using a simple furnace according to this method. Therefore, the flux method has been used for the growth of many kinds of single crystals.
Generally, in the flux method, the step of extracting a grown single crystal from the fluid or solidified flux is required after the completion of the crystallization. The extracting methods conventionally employed include:
(1) a method of picking up a single crystal by crushing the solidified flux with a hammer, PA1 (2) a method of extracting a single crystal by dissolving away the flux with a solvent suitable for the flux used, such as hot hydrochloric or, nitric acid or hot water, and PA1 (3) a method of extracting a single crystal from the flux by overflowing it by slanting a crucible before the solidification of the flux.
Here, it is necessary to select such a material as flux that only a sufficient amount of raw materials is dissolved at a high temperature and that a solid solution of the flux with the raw materials is not formed in the solid phase. In addition, it is also required to select such a flux as to be used at such a temperature and in such an atmosphere as to allow the raw materials to remain stable.
Examples of the fluxes conventionally used for the growth of single oxide crystals include PbO, PbO-PbF.sub.2, B.sub.2 O.sub.3, Bi.sub.2 O.sub.3 and KF, which were empirically found to be suitable for such a flux. Of those flux materials, Bi.sub.2 O.sub.3, PbO or the like is shrunk and hardened upon solidification. When a flux of such properties is used, a grown single crystal receives such a pressure applied thereto upon solidification of the flux that the single crystal is disadvantageously subject to the development of cracks in the case of the above-mentioned method (1) or (2). Further, the above-mentioned method (3) is seldom employed, because it involves such demerits that the overflown flux adheres to the furnace material and dissolves the furnace material, and that expensive raw materials are disposed every time after the production of a single crystal.
On the other hand, in order to grow a single crystal of bismuth-substituted gadolinium-iron garnet, it has been reported that Bi.sub.2 O.sub.3 is used as flux (see "Faraday Rotation and Optical Absorption of a Single Crystal of Bismuth-Substituted Gadolinium Iron Garnet" by H. Takeuchi, S. Ito, I. Mikami and S. Taniguchi, J. Appl. Phys., Vol. 44, No. 10 (1973), pp. 4789-4790). In this paper the, conditions of growing such a single crystal have not being clearly disclosed. If such a single crystal is grown by the usual flux method, a grown single crystal receives a pressure from the solidified flux and is subject to the development of cracks, as mentioned above. The crystal having bismuth uniformly substituted therein could be obtained.